An index case is “the first documented case of an infectious disease or genetically transmitted condition or mutation in a population, region, or family.” It may also, however, refer to an individual who has a disease, condition, or mutation that is the first one identified in a population.
This second sense is synonymous with index patient.
A related term is patient zero, “a person identified as the first to become infected with an illness or disease in an outbreak.” Patient zero is especially used to refer to a person documented as being the first known case of a communicable disease in a particular population or region.
[출처] www.merriam-webster.com/words-at-play/coronavirus-words-guide/covid-19
